1. Field of the Invention
The present invention relates to a technology for controlling flying characteristics of liquid or a position to which liquid is delivered and to liquid ejecting device and method in which liquid in liquid cell is ejected from a nozzle. The present invention specifically relates to, in liquid ejecting device including heads each having a plurality of liquid ejecting portions arranged in parallel and liquid ejecting method using the heads each having the ejecting portions arranged in parallel, a technology for controlling a direction (a direction in which liquid is delivered) in which liquid is ejected from each liquid ejecting portion.
2. Description of the Related Art
Inkjet printers have been conventionally known as a type of liquid ejecting device including heads which each have a plurality of liquid ejecting portions arranged in parallel. A thermal method that uses thermal energy to eject ink is known as one of ink ejecting methods for inkjet printers.
In an example of printer-head chip structure using the thermal method, ink in an ink cell is heated by a heating element disposed in the ink cell to produce bubbles in the ink on the heating element, and the energy of the production of the bubbles ejects the ink. A nozzle is formed in the upper side of the ink cell. When the bubbles are produced in the ink in the ink cell, the ink is ejected from the ejecting outlet of the nozzle.
From the viewpoint of head structure, there are two methods, a serial method and a line method. In the serial method, an image is printed by moving a printer-head chip in the width direction of printing paper. In the line method, many printer-chip heads are arranged in the width direction of printing paper to form a line head for the width of the printing paper.
FIG. 18 is a plan view showing a line head 10 of the related art. Although four printer-head chips 1 (N−1, N, N+1, and N+2) are shown in FIG. 18, actually, more printer-head chips are arranged.
In each printer-head chip 1, a plurality of nozzles 1a having ejecting outlets for ejecting ink are formed. The nozzles la are arranged in parallel in a given direction, and the given direction is identical to the width direction of the printing paper. Also, the printer-head chips 1 are arranged in the given direction. Adjacent printer-head chips 1 are arranged so that their nozzles 1a oppose each other, and in a portion in which two printer-head chips 1 are adjacent to each other, the pitch of the nozzles 1a is consecutively maintained (see the detail of portion A in FIG. 18).
The related art shown in FIG. 18 has the following problems.
When ink is ejected from the printer-head chips 1, it is ideal that the ink is ejected perpendicularly to the ejection surface of the printer-head chips 1. However, various factors may cause a case in which an angle at which the ink is ejected is not perpendicular.
For example, when a nozzle sheet having the nozzles 1a formed thereon is bonded to the upper side of ink cells having heating elements, the problem is that positional shifting occurs between pairs of the ink cells and the heating elements, and the nozzles 1a. When the nozzle sheet is bonded so that the center of the nozzles 1a is positioned in the center of the ink cells and the heating elements, the ink is ejected perpendicularly to the ink ejection surface (the nozzle sheet surface). However, if a shift occurs between the ink cells and the heating elements, and the nozzles 1a, the ink cannot be ejected perpendicularly to the ejection surface.
Also, a positional shift can occur due to a difference in thermal expansion factor between the pairs of the ink cells and the heating elements, and the nozzle sheet.
It is assumed that, when the ink is ejected perpendicularly to the ejection surface, an ink droplet is delivered to an ideally exact position. When the angle of ejection of the ink is shifted from perpendicularity by θ, positional shift ΔL in delivery of ink droplet isΔL=H×tan θwith the distance (normally 1 to 2 millimeters in the case of the inkjet method) between the ejection surface and the surface (a surface on which the ink droplet is delivered) of printing paper set to H (H is constant).
When such a shift in angle of ejection of the ink occurs, in the serial method, the shift in angle appears as a shift in delivery of ink between two nozzles 1a. In the line method, in addition to the shift in delivery of ink, the shift in angle appears as a positional shift in delivery between two printer-head chips 1.
FIGS. 19A and 19B are respectively a section view and plan view showing the state of printing performed by the line head 10 (in which the printer-head chips 1 are arranged in parallel in a direction in which the nozzles 1a are arranged) shown in FIG. 18. In FIGS. 19A and 19B, assuming that printing paper P is fixed, the line head 10 does not move in the width direction of the printing paper P, and performs printing while moving from top to bottom of the plan view in FIG. 19B.
In the section view in FIG. 19A, among the line head 10, three printer-head chips 1, that is, the N-th printer-head chip 1, the (N+1)-th printer-head chip 1, and the (N+2)-th printer-head chip 1 are shown.
As shown in the section view in FIG. 19A, in the N-th printer-head chip 1, ink is slantingly ejected in the left direction as is indicated by the left arrow. In the (N+1)-th printer-head chip 1, ink is slantingly ejected in the right direction as is indicated by the central arrow. In the (N+2)-th printer-head chip 1, ink is perpendicularly ejected without a shift in angle of ejection as is indicated by the right arrow.
Accordingly, in the N-th printer-head chip 1, the ink is delivered, being off to the left from a reference position, and in the (N+1)-th printer-head-chip 1, the ink is delivered, being off to the right from the reference position. Thus, between both, the ink in the N-th printer-head chip 1 and the ink in the (N+1)-th printer-head chip 1 are delivered to opposite directions. As a result, a region in which no ink is delivered is formed between the N-th printer-head chip 1 and the (N+1)-th printer-head chip 1. In addition, the line head 10 is only moved in the direction of the arrow in the plan view in FIG. 19B without being moved in the width direction of the printing paper P. This forms a white stripe B between the N-th printer-head chip 1 and the (N+1) printer-head chip 1, thus causing a problem of deterioration in printing quality.
Similarly to the above case, in the (N+1)-th printer-head chip 1, the ink is delivered, being off to the right from the reference position. Thus, the (N+1)-th printer-head chip 1 and the (N+2)-th printer-head chip 1 have a common region in which the ink is delivered. This causes a discontinuous image and a stripe C which has a color thicker than the original color, thus causing a problem of deterioration in printing quality.
When such a shift in a position to which ink is delivered occurs, the degree to which a stripe looks noticeable depends on an image to be printed. For example, since a document or the like has many blank portions, a stripe will not look noticeable if it is formed. Conversely, in the case of printing a photograph image in almost all the portions of printing paper, if a slight strip is formed, it will look noticeable.
For the purpose of preventing the formation of such a stripe, Japanese Patent Application No. 2001-44157 (hereinafter referred to as “Earlier Application 1”) has been filed by the Assignee of the present Patent Application. In the invention of Earlier Application 1, a plurality of heating elements (heaters) which can separately be driven are provided in ink cells, and by separately driving the heating elements, a direction in which each ink droplet is ejected can be changed. Accordingly, it has been thought that the formation of the above stripe (the white stripe B or the stripe C) can be prevented by the Earlier Application 1.
However, although Earlier Application 1 deflects the ink droplet by separately controlling the heating elements, the result of further study by the present Inventors has indicated that, when the method of Earlier Application 1 is employed, the ejection of the ink droplet may become unstable and a printed image having high quality cannot stably be obtained. The reason is described below.
According to study by the present Inventors, as described in PCT/JP/08535 (hereinafter referred to as “Earlier Application 2”) filed by the Assignee of the present Application, normally, the quantity of ejection of ink from nozzles does not increase in a monotone in accordance with an increase in power applied to heating elements, but tends to rapidly increase when the power exceeds a predetermined value (see Earlier Application 2, page 28, lines 14 to 17, and FIG. 18). In other words, a sufficient quantity of ink droplet cannot be ejected unless power equal to the predetermined value or greater is supplied.
Therefore, in the case of separately driving the heating elements, when ink droplets are ejected by performing only driving of only some heating elements, a sufficient amount of heat for ink droplet ejection must be generated only by the driving. Accordingly, in the case of separately driving the heating elements, when some heating elements are used to eject the ink droplets, power supplied to the heating elements must be increased. This situation causes a disadvantageous situation to size reduction in the heating element which is associated with increased resolution in the recent years.
In other words, in order to perform stable ejection of ink droplets, the amount of generated energy per unit area of each heating element must be extremely increased than usual. As a result, damage to small-sized heating elements is enhanced. This shortens the life of the heating elements, thus shortening the life of the head.
The above problems are similarly found in the case of using the technologies described in Japanese Patent No. 2780648 (hereinafter referred to as “Earlier Application 3”) and Japanese Patent No. 2836749 (hereinafter referred to as “Earlier Application 4”).
Although Earlier Application 3 discloses an invention for preventing a satellite (scattering of ink), and Earlier Application 4 discloses an invention for the purpose of realizing stable control of gradations, both are similar to Earlier Application 1 in using a plurality of heating elements and separately driving the heating elements.
By driving some heating elements among a plurality of heating elements to eject ink droplets, as in Earlier Applications 3 and 4, the ink droplets can be ejected and deflected as described in Earlier Application 3, or gradation control can be performed as described in Earlier Application 4. However, in the case of using provided heating elements which are small-sized in association with increased resolution in the recent years, when only some heating elements are driven to eject ink droplets, the supply to them of power enabling stable ejection causes a problem of a decrease in the life of the heating elements.
In the invention in Earlier Application 4, an increase in the amount of power to each heating element represents an increase in the minimum quantity of ink droplet. Thus, it is difficult to perform gradation control which is the original object of the invention in Earlier Application 4.
Conversely, in Earlier Application 4, when the amount of power supplied to each heating element is reduced, there is a possibility that the ink droplets cannot stably be ejected, as described above.
As is understood from the above description, in the case of using a head including heating elements which are small-sized in association with increased resolution, it is impossible to prevent the formation of the above stripes by the related art and the technologies in Earlier Applications 1 to 4.